Batteries are used in downhole applications to provide power to equipment, such as logging instruments, which gather and relay information to the surface regarding conditions below. A typical conventional battery usually consists of multiple cells connected in series to form a battery or “stick.” Cells are used cumulatively in the battery to provide the desired output voltage. Depending upon the power requirements and available internal spaces of the downhole tool, multiple batteries are sometimes used.
However, conventional battery deployments have several operating disadvantages. For example, the current drawn from batteries may exceed the battery's design current density (amps per unit area of electrode) limit. In some batteries, such as lithium batteries, exceeding the rated current density results in an irreversible reaction, which causes the loss of some electrode material. Under these conditions, the amp-hour capacity of the battery drops from its design value, and total battery life decreases accordingly.
One conventional method to attempt to avoid such disadvantages is to deploy multiple sticks in parallel with diode isolation. While this multiple stick arrangement reduces the current demand on each stick, there remains another set of disadvantages. In this configuration, the batteries discharge at the same time, and substantially the same rate, even when a single battery could provide sufficient current to the equipment. Thus, the parallel-connected batteries' amp-hour capacity degrades simultaneously.
This operating mode is also known to result in unnecessary waste and costs of battery disposal. Due to the costs involved with retrieving tools from downhole in order to change batteries, it is often not economically viable to re-use partially discharged batteries below a certain charge level. For example, two batteries wired in parallel may each be discharged by sixty percent, which in some conditions may render them both insufficient for re-use later. Disposal of both would then be required, wasting the unused forty percent of each battery's remaining rated life.
Another troublesome property of lithium batteries is the phenomena of battery electrode passivation wherein the battery output impedance rises and makes available battery power fall.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.